When two wires composed of dissimilar metals are joined at both ends, and one of the ends is heated, a continuous current flows in the wires to establish a thermoelectric circuit. If the circuit is open, the open circuit voltage, or "thermocouple voltage", is a function of the junction temperatures and the composition of the two metals. This principal is the basis of thermocouple type temperature measurement systems, wherein a junction is exposed to a temperature to be measured, and the voltage of the junctions is measured by a voltmeter to obtain a reading that is related, depending upon the type of thermocouple involved, to the measured temperature.
Thus, with reference to FIG. 1, thermocouple 10 is formed of a pair of dissimilar metal wires 12, 14 joined together at a junction 16, and with the open ends 18, 20 connected to a pair of circuit wires 22, 24 (of the same conductive material) to be measured by voltmeter 26.
The voltage measured at circuit wires 22, 24 appears, by inspection, to be equal to the voltage generated by the dissimilar metal junction 16. Upon closer analysis, however, this is not the case. Assume, for example, that thermocouple 10 is a copper-constantan (type T) thermocouple having wire 12 formed of copper (Cu) and wire 14 formed of constantan (C). Assume further that circuit wires 22, 24 as well as the wires of voltmeter 26 are formed of copper. Bearing in mind that a thermocouple is formed by junctions of dissimilar metal wires, junction 20 is a copper-constantan junction whereat an additional thermocouple voltage is developed. Junction 18 formed by copper wires 22, 24, however, does not establish a thermocouple voltage. The voltage measured by voltmeter 26 at wires 22, 24 is therefore not the voltage generated by junction 16, but rather is the sum of the voltage generated by junction 16 and the voltage generated by junction 20. Stated another way, the voltage generated by junction 20 must be subtracted from the voltage measured by voltmeter 26 to obtain the voltage generated by junction 16 and thereby determine the measured temperature. The junction 20 is referred to hereinafter as a "reference junction".
To determine the voltage of reference junction 20, the temperature of the junction must be known. As one technique used to enable the temperature of any reference junction to be determined, the junction is immersed into an ice bath, forcing the reference junction to have a fixed temperature of 0.degree. C. The reference junction voltage at this temperature is obtained from standard tables. The voltage is subtracted from the output voltage measured by the voltmeter during each reading to obtain a voltage related directly to the temperature of junction 16.
As mentioned, because the additional junction 18 is a copper-copper junction, there is no voltage generated thereat to be taken into account in the output voltage measured by voltmeter 26. In a more general case, however, dissimilar metal junctions may be established both at junctions 18 and 20. In FIG. 2, for example, thermocouple 28 is an iron-constantan (type J) thermocouple with an iron (Fe) wire 32 and a constantan (C) wire 34 joined together at junction 30. Circuit wires 22, 24 again are made of copper. There is thus an iron-copper junction at 19 and a constantan-copper junction at 21. Assume further, however, that, as is more typical, the iron-constantan thermocouple 28 is connected to a pair of iron wires 40, 42, in turn connected to the copper circuit wires 22, 24 as shown in FIG. 3. There is, of course, no dissimilar metal junction formed at the junction 45 of iron wires 32 and 40. A dissimilar metal junction is, however, formed at the junction 44 between constantan wire 34 and iron wire 42, and iron-copper junctions also are formed at junctions 19 and 46.
Because junctions 19 and 46 are in series opposition to each other, and both are formed by the same pair of dissimilar metals, the voltages generated by the junctions will cancel if the temperatures of them are the same. The output voltage measured by voltmeter 26 at circuit wires 22 and 24 thus is substantially equal to the difference between the voltages generated by junctions 30 and 44; the voltage of junction 30 and thereby the measured temperature is, again, proportional to the difference between the voltage measured by voltmeter 26 and the voltage of the reference junction 44. Accordingly, if the temperature at junction 44 is known, the temperature at 28, independent of junctions 19 and 46, can be determined by the voltmeter 26.
To insure that junctions 19 and 46 are maintained at the same temperature, whereby the voltages at junctions 19 and 46 cancel, the two junctions are housed within an isothermal block 48, as shown in FIG. 4 (because no dissimilar metal junction is formed between iron wires 32 and 40, junction 45 is omitted from FIG. 4). To eliminate the need for a reference junction ice bath, the junction 44 is also located within the isothermal block 48 whereat the temperature of the reference junction will be measured. A temperature sensor 50 located within the isothermal block 48 measures the temperature of the isothermal block 48 as well as of reference junction 44 through stored table look up or other conventional means, the reference voltage to be subtracted from the voltage at voltmeter 26 to obtain the voltage at and thereby the temperature of junction 30 is determined.
The isothermal block 48 must be electrically insulating and have a very high thermal capacity or mass to cause all three junctions 19, 44 and 46 to be at the same temperature. This makes up for any poor thermal connection from the isothermal block 48 to temperature sensor and sensor wires by maintaining the thermocouple to be at an equitemperature. The block 48 thereby enables the temperatures of the junctions to track each other and reach thermal equilibrium in as short a time as possible. The isothermal block 48 must further retain the reference junction and sensor to a circuit board carrying additional signal processing circuitry, and the block and circuitry must be packaged compactly.
One object of the invention, therefore, is to provide an isothermal block that has a high thermal conductivity.
Another object is to provide an isothermal block that retains at least one reference junction and a junction temperature sensor to the surface of a printed circuit board.
Another object is to provide an isothermal block and reference junction temperature sensor structure that is compact, easily manufactured and mounted conveniently to a circuit board.